Improve search result quality.

This should fix issues #197 and #194.

We no longer give so much weight to the number of downloads.
Previously, the number of downloads could change an item's search
score by a factor of about 500,000x (for `ring-core`, with
approx. 500K downloads).  This meant that a search that had any terms
in common with popular items almost always returned those items at the
top, even if there was an item that had a perfect text match.

With this change, `ring-core`'s downloads would only give it a
multiplier of about 2.43x.

Here's a look at the the first 5 items returned by various searches, comparing the old code to the new code.

### Search for 'lein-garden'

Old:

|Name | Description |
|-----|-------------|
|lein-ring 0.8.10      |Leiningen Ring plugin|
|lein-cljsbuild 1.0.3  |ClojureScript Autobuilder Plugin|
|lein-midje 3.1.3      |Run Midje and clojure.test tests|
|lein-newnew 0.3.7     |A Leiningen plugin for generating new projects based on templates.|
|lein-cljsbuild/cljs-compat 1.0.0-SNAPSHOT  |Matrix of cljsbuild/ClojureScript compatibility|

New:

|Name | Description |
|-----|-------------|
|lein-garden 0.1.8|A Leiningen plugin for automatically compiling Garden stylesheets|
|org.clojars.nbeloglazov/lein-garden 0.1.0-SNAPSHOT|Lein plugin for compiling garden files to css.|
|lein-describe 0.2.0|A Leiningen plugin for displaying detailed project information.|
|garden-watch 0.1.7|Garden source to CSS watcher and converter|
|malcolmsparks/garden 0.1.0-beta|A library for generating CSS in Clojure.|

`lein-describe` shows up because it incorrectly lists its repository
as https://github.com/noprompt/lein-garden

### Search for 'lein-modules'

Old:

|Name | Description |
|-----|-------------|
|lein-ring 0.8.10|Leiningen Ring plugin|
|lein-cljsbuild 1.0.3|ClojureScript Autobuilder Plugin|
|lein-midje 3.1.3|Run Midje and clojure.test tests|
|org.apache.hadoop/hadoop-core 0.20.2-dev|Hadoop is the distributed computing framework of Apache; hadoop-core contains the filesystem, job tracker and map/reduce modules|
|lein-newnew 0.3.7|A Leiningen plugin for generating new projects based on templates.|

New:

|Name | Description |
|-----|-------------|
|lein-modules 0.3.1|Similar to Maven multi-module projects, but less sucky|
|modules|
|modules|
|modules|
|modules|

Those bogus `modules` items are due to issue #132.

### Search for 'lein-ring'

Old:

|Name | Description |
|-----|-------------|
|ring/ring-core 1.3.0-beta2|Ring core libraries.|
|ring/ring-servlet 1.3.0-beta2|Ring servlet utilities.|
|ring/ring-jetty-adapter 1.3.0-beta2|Ring Jetty adapter.|
|ring/ring-devel 1.3.0-beta2|Ring development and debugging libraries.|
|lein-ring 0.8.10|Leiningen Ring plugin|

New:

|Name | Description |
|-----|-------------|
|lein-ring 0.8.10 89902|Leiningen Ring plugin|
|com.andrewmcveigh/lein-ring 0.8.3-SNAPSHOT 59|Leiningen Ring plugin|
|org.clojars.strongh/lein-ring 0.8.7 16|Leiningen Ring plugin|
|org.clojars.abbot/lein-ring 0.8.7 5|Leiningen Ring plugin|
|jarohen/lein-ring 0.8.4-SNAPSHOT 3|Leiningen Ring plugin|



### Search for 'compojure'

Just to show that we still do well on popular items.

Old:

|Name | Description |
|-----|-------------|
|compojure 1.1.8|A concise routing library for Ring|
|compojure/lein-template 0.3.4|Compojure project template for Leiningen|
|compojure-app/lein-template 0.4.2|Compojure project template for Leiningen|
|shoreleave/shoreleave-remote-ring 0.3.0|A smarter client-side with ClojureScript : Ring- (and Compojure-) server-side Remotes support|
|sandbar 0.3.3|Clojure web application libraries built on top of Ring and Compojure.|

New:

|Name | Description |
|-----|-------------|
|compojure 1.1.8|A concise routing library for Ring|
|org.clojars.ctdean/compojure 0.3.1||
|org.clojars.tomo/compojure 0.3.1||
|metosin/compojure-api 0.11.3|Compojure Api|
|compojure-throttle 0.1.5|Throttling middleware for compojure|