Minor source code changes required to compile with Microsoft Visual C++ 11 (2012)

When compiling under Microsoft Visual C++ 11, there are a few cases where some boost
function names are in scope at the same time as std:: functions with the same names.

I don't understand why this isn't a problem for every C++ toolchain.  Nevertheless, a few 
explicit namespace qualifiers in the source code solves the problem when building with MSVC.

I also had to add an implementation for strptime, which does not exist on Windows.  I used the implementation from http://googlesitemapgenerator.googlecode.com/svn/trunk/src/common/timesupport.cc, which is Apache 2.0 licensed.  The source code is #ifdef bracketed with WIN32, so it is not compiled into other targets.

I have also re-built and re-tested ledger under Ubuntu 13.04 with these source code changes.  The code still compiles, and the resulting executable still works.

I cannot test any other platforms.