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!
Added new traceException function
Fixed script evaluation bug in FAKE.Deploy
FAKE.Deploy switched to FCS
Fix for https://github.com/fsharp/FAKE/issues/461
Reference to FSharp.Compiler.Service has been updated to v0.0.59
(that fix https://github.com/fsharp/FSharp.Compiler.Service/pull/184)
Also https://github.com/fsharp/FAKE/issues/501 should be fix
Add HipChat notification helper
Merge pull request #519 from sergey-tihon/master

FAKE.Deploy switched to FCS
Merge pull request #520 from sergey-tihon/workDir

FAKE.Deploy WorkDirectory fix #461
Merge pull request #523 from ololabs/hipchat

Add HipChat notification helper
Using JSON.NET 6.0.4
Using Nuget 2.8.2
Using JSON.NET 6.0.4