Don't call external uname when $OSTYPE will do (#1911)

* lib/helpers: use `$OSTYPE` instead of `$(uname)`

* plugins/osx: use `$OSTYPE` instead of `$(uname)`

* plugins/boot2docker: use `$OSTYPE` instead of `$(uname)`

* plugins/python: use `$OSTYPE` instead of `$(uname)`

* plugins/base: use `$OSTYPE` instead of `$(uname)`

Alsö, use `[[` instead of `[` as the former has less insane argument handling being shell syntax rather than a builtin command that must emulate being a real binary

* completion/brew: use `$OSTYPE` instead of `$(uname)`

* completion/git: use `$OSTYPE` instead of `$(uname)`

Alsö, use `[[` instead of `[`.

* completion/fabric: use `$OSTYPE` instead of `uname`

* theme/demula: use `$OSTYPE` instead of `$(uname)`

* theme/rana: use `$OSTYPE` instead of `$(uname)`

plugins/available/python.plugin.bash

@@ -1,7 +1,7 @@
 cite about-plugin
 about-plugin 'alias "shttp" to SimpleHTTPServer'
 
-if [ $(uname) = "Linux" ]
+if [[ "$OSTYPE" == 'linux'* ]]
 then
   alias shttp='python2 -m SimpleHTTPServer'
 else
@@ -16,7 +16,7 @@ function pyedit() {
 
     xpyc=`python -c "import os, sys; f = open(os.devnull, 'w'); sys.stderr = f; module = __import__('$1'); sys.stdout.write(module.__file__)"`
 
-    if [ "$xpyc" == "" ]; then
+    if [[ "$xpyc" == "" ]]; then
         echo "Python module $1 not found"
         return -1