Pluginsv2

Change format of returned source tree list from a string array to the following (suggested by @mtholder):
```json
{
  "root_node_id" : 132,
  "study_list" : [ {
    "study_id" : "pg_2827",
    "git_sha" : "a2c48df995ddc9fd208986c3d4225112550c8452",
    "tree_id" : "6577"
  }, {
    "study_id" : "pg_761",
    "git_sha" : "a2c48df995ddc9fd208986c3d4225112550c8452",
    "tree_id" : "1415"
  },
[...]
 {
    "study_id" : "pg_2849",
    "git_sha" : "a2c48df995ddc9fd208986c3d4225112550c8452",
    "tree_id" : "6615"
  }, {
    "study_id" : "pg_2822",
    "git_sha" : "a2c48df995ddc9fd208986c3d4225112550c8452",
    "tree_id" : "6569"
  }, {
    "study_id" : "taxonomy",
    "git_sha" : "",
    "tree_id" : ""
  } ],
  "root_taxon_name" : "cellular organisms",
  "num_source_studies" : 484,
  "taxonomy_version" : "2.8_5",
  "root_ott_id" : 93302,
  "num_tips" : 2339460,
  "date" : "Sun Sep 14 01:27:06 EDT 2014",
  "tree_id" : "otol.draft.22"
}
```
There is an optional argument ```list_sources``` which defaults to ```true```.

I've talked to @jar398 about maybe making this a separate service in the future, but at the moment it is just a better version of the existing service.

Would you like to merge @mtholder?