Specify minimum versions in requirements.txt.

We used to use requirements.txt for frozen dependency testing in Travis-CI.
Given that pecan is a framework, it probably makes more sense to specify
"minimum" versions.  This change extracts the list of minimum dependencies into
a requirements file that can be used in other places, like documentation and
installation tools.

Change-Id: Ie2fef9fd2e5ed261b4cc82139882b226ba787680

requirements.txt

@@ -1,5 +1,5 @@
-Mako==0.6.2
-MarkupSafe==0.15
-WebOb==1.2b3
-WebTest==1.3.3
-simplegeneric==0.8.1
+WebOb>=1.2dev
+Mako>=0.4.0
+WebTest>=1.3.1
+simplegeneric>=0.8.1
+six

setup.py

@@ -7,13 +7,11 @@ version = '0.3.2'
 #
 # determine requirements
 #
-requirements = [
-    "WebOb >= 1.2dev",  # py3 compat
-    "simplegeneric >= 0.8",  # py3 compat
-    "Mako >= 0.4.0",
-    "WebTest >= 1.3.1",  # py3 compat
-    "six"
-]
+with open('requirements.txt') as reqs:
+    requirements = [
+        line for line in reqs.read().split('\n')
+        if (line and not line.startswith('-'))
+    ]
 
 try:
     import json  # noqa