Plugin cleanup and refactor

[josh146]

Description of changes:

This is quite a significant clean up and refactor of the plugin. In particular,

• All code blocks not being used were removed

• Superfluous conditionals were deleted

• Unused methods and attributes were deleted

• Documentation and licenses were updated to bring them up to standard

• Long and confusing names were shorted

• device.py was split into separate files, one per device

• The long standing IBM Q bug (see Bug: Missing url for qiskit #38 and added ibmqx_url as additional option to IBMQiskitDevice #39) was fixed, and the IBM Q device now supports both v1 and v2 of the IBM API

• The deprecated custom gates in qiskitops.py were removed, and replaced with the recommended approach to defining Qiskit custom gates.

To-do: None of the tests were refactored, this is still on the todo list.

[codecov]

Codecov Report

Merging #44 into master will increase coverage by 4.61%.
The diff coverage is 96.68%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
+ Coverage   92.35%   96.96%   +4.61%     
==========================================
  Files           6        8       +2     
  Lines         301      264      -37     
==========================================
- Hits          278      256      -22     
+ Misses         23        8      -15

Impacted Files	Coverage Δ
pennylane_qiskit/_version.py	100% <100%> (ø)	⬆️
pennylane_qiskit/aer.py	100% <100%> (ø)
pennylane_qiskit/ops.py	100% <100%> (ø)	⬆️
pennylane_qiskit/__init__.py	100% <100%> (ø)	⬆️
pennylane_qiskit/basic_aer.py	100% <100%> (ø)
pennylane_qiskit/ibmq.py	85.18% <85.18%> (ø)
pennylane_qiskit/gates.py	93.47% <93.47%> (ø)
pennylane_qiskit/qiskit_device.py	99.2% <99.2%> (ø)
... and 3 more

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[quantshah]

I made some small comments on typos. I could update them myself.