Add support for inch(in) and millimeter(mm) dimensions in dimens.xml

Moved the changes off my master branch so had to resubmit the pull!

When <code>com.xtremelabs.robolectric.res.ResourceLoader</code> encounters a dimens.xml such as the following:

<code>
&lt;?xml version=&quot;1.0&quot; encoding=&quot;utf-8&quot;?&gt;
&lt;resources&gt;
    &lt;dimen name=&quot;text_size&quot;&gt;19in&lt;/dimen&gt;
&lt;/resources&gt;
</code>

<code>java.lang.RuntimeException: java.lang.NumberFormatException: For input string: "19in"
at com.xtremelabs.robolectric.res.ResourceLoader.init(ResourceLoader.java:148)</code>

<code>
&lt;?xml version=&quot;1.0&quot; encoding=&quot;utf-8&quot;?&gt;
&lt;resources&gt;
    &lt;dimen name=&quot;text_size&quot;&gt;86mm&lt;/dimen&gt;
&lt;/resources&gt;
</code>

an exception is thrown:

<code>java.lang.RuntimeException: java.lang.NumberFormatException: For input string: "86mm"
at com.xtremelabs.robolectric.res.ResourceLoader.init(ResourceLoader.java:148)</code>

This pull request adds support for the Inch ("in") and Millimeter ("mm") dimension unit of measure:

http://developer.android.com/guide/topics/resources/more-resources.html#Dimension

Mirroring this request that added point("pt")

https://github.com/pivotal/robolectric/pull/321

(deprecated pull https://github.com/pivotal/robolectric/pull/364)