Change variable names to symbols

[dpsanders]

• Use :x instead of "x"
• Provide a Dict to look up which number variable a given symbol corresponds to.

[lbenet]

I don't mind the first option.

Regarding the second one, I think we can get it directly as look_up(:x) with the function

lookup(s::Symbol) = find(v -> v==s, TaylorSeries._params_TaylorN_.variable_symbols)

It would return a vector with the index where the symbol is found, or a zero-length vector, if the symbol doesn't exist.

[lbenet]

This is only partially addressed by #154, shall we close this, or keep it open for the future?

[lbenet]

Closing, since this is somehow provided by #154 and #155