* fixes broken msvc build
* smaller xml files for testing serialization
* fixing many warnings on msvc
* bug-fixes to mp_int regarding operators and new tests to check their behavior
* don't expect iterable histogram::at to have size method
* build benchmarks in release mode
* test everything by default when b2 is run on project directory
* more test coverage
* Travis uses b2 and codecov now
* replacing boost::multiprecision with custom implementation to avoid the dependency
* improved axis implementation: `update` is now a normal method
* introduced use_default tag type to set defaults
* whitespace fixes and doc update
