change font in build view to monospace

compilers like clang and rustc annotate errors with symbols to show where the error lies.
without a monospace font this is impossible to read

e.g.

```
D:\rust\src\librustdoc\lib.rs:19 #![feature(globs, struct_variant, macro_rules, phase, slicing_syntax, tuple_indexing)]
                                                   ^~~~~~~~~~~~~~
```
Merge pull request #18 from oli-obk/patch-1

change font in build view to monospace