Extract, move and test StrToInt64

This basically breaks down #173.

- Rename strtToInt64 to StrToInt64
- Moved into new files /mastercore/parse_string.h and /mastercore/parse_string.cpp
- Adopted and converted @zathras-crypto's tests to BOOST tests which can be run via ./src/test/test_bitcoin
- Namespace "mastercore" is used
- I tend to let this sit in it's own file, because it's only used in mastercore_rpc and sendmpdialog whereby Format__MP is used in other places

Build and practical tests done.