Allow Windows MSI Bundler productVersion to differ from the general version

Currently the version of a MSI bundle must be the same version used for other bundlers, such as the EXE bundler. The problem is that MSI requires a very restrictive numbering format of "1.2.3" whereas sometimes a more liberal versioning scheme is useful, such as "1.2.3_beta_win32_20140604_debug".

We will introduce a bundler argument "win.msi.productVersion" that can be set to the shorter version, and the default will be to use the version for the whole build. This version will continue to be validated.