Fix UnsatisfiedLinkError when path to plugin contains spaces.

Convert URL to URI for decoding.
URI will decode entities when retrieving the path, something URL won't do.  This makes a difference when the location was something like `file:/C:/Program%20Files/Jenkins/`.

Manual testing:
1. Created a Windows 7 x64 VM.
2. Downloaded Jenkins 1.536 for Windows and ran the installer, picking defaults.
3. Installed the TFS plugin version 3.0.0.
4. Created a job that used TFS as the SCM.
5. Ran the job
5.1. The job failed with an `UnsatisfiedLinkError`
5.2. The value of the `com.microsoft.tfs.jni.native.base-directory` property looked like `C:\Program%20Files%20(x86)\Jenkins\plugins\tfs\WEB-INF\lib\native`
6. Upgraded the TFS plugin to version 3.0.1-SNAPSHOT by restarting Jenkins.
7. Ran the job again.
7.1. The job suceeeded.
7.2. The value of the `com.microsoft.tfs.jni.native.base-directory` property looked like `C:\Program Files (x86)\Jenkins\plugins\tfs\WEB-INF\lib\native`