Bugfix for various strtol/strtoll issues 

This fixes a number of issues in the existing strtoll/strtol (and variants) implementation. Notably, this fixes issues when trying to parse hex numbers (0x...) with an undefined (=0) base where strtoll/strtol will (according to spec) resort to the string representation to identify the base. 

A number of test cases for various bases (2, 8, 10, 16) is included.

Generally, we experience these sort of issues in "corner cases" fairly often with the existing stdlib implementation in library.js. In light of asm.js being so fast, I wonder whether emscripten wouldn't be better of by replacing most of that with an existing C library which has matured over time?