oft: Add support for XML output

Add support for xUnit-format XML output by utilising the 'xmlrunner'
package. This feature is optional, and is disabled by default.

Resolves issue #123.

Signed-off-by: Stephen Finucane <stephenfinucane@hotmail.com>
oft: Move profiler setup/teardown to function

Profiler setup/teardown was done "in-situ". This was inconsistent
with the general design of 'oft'. Fix this, by moving these
operations to a seperate file.

In addition, add a '--profile-file' parameter, so that the output
file can be changed if necessary.

Signed-off-by: Stephen Finucane <stephenfinucane@hotmail.com>