Rename _build_index to build_byte_index

[levitsky]

Fixes #140.

• unified method name is now build_byte_index across all indexed and index-saving parsers;
• the method returns the index rather than assign it (the behavior of build_byte_index is preserved);
• warning is thrown when defining an IndexedXML subclass with a _build_index method (Python 3 only).

@mobiusklein could you please take a look at it?

[mobiusklein]

There is a small change that I'll make in my downstream code to account for https://github.com/levitsky/pyteomics/pull/142/files#diff-7326fe871ea5482e4c336e02fabaa716957ab8aa12670f129c09d95d26630cceR538, but this looks good to me.