Add pushd/popd to FileUtils

Adds familiar pushd and popd commands to better support recursive
directory navigation.
Merge pull request #513 from brianary/PushdPopd

Add pushd/popd to FileUtils
Bump version to 3.2.15
NuGet: support fallback framework groups

Since NuGet 2.0, dependencies and references can be specified by target
framework, where only the entries from the matching group are considered.
NuGet also allows defining a fallback group where the targetFramework is
missing, which is used if none of the other groups match.

Previously FAKE did emit the empty targetFramework attribute, which causes
NuGet to complain. This change skipts the attribute if its value is null
or empty, in compliance with the nuget specs.
Merge pull request #514 from cdrnet/nuget-fallbackFrameworkGroup

NuGet: support fallback framework groups
Bump version to 3.2.16
Don't crash during tool discovery - #167
Bump version to 3.2.17
Another happy FAKE user as reference!
Merge pull request #518 from ilkerde/master

Another happy FAKE user as reference!