Merge pull request #141 from doowb/master

Updates to references to href helper and validating ext option.
Added a variable for excluding pages from build

With this you can use frontmatter to disable rendering of a page when needed. (Useful for prototyping)

I explained the justification for this in issue #122

The actual implementation may need some finesse still.
Added an option to remove extra whitespace around hbs tags

It annoys me and apparently many others that handlebars does not strip extra whitespace around its elements when compiling. 

wycats/handlebars.js#336

This is a quick hack to make it strip that extra whitespace as if the rows with only a single hbs tag were not separate rows. This pull possibly has some bugs in its regexp, but it's a start.

I thought that maybe this should be toggleable by the user, so only by adding an option: removeHbsWhitespace:true to grunt config of assemble will enable this feature. (without it, no ill effects should happen)

With removeHbsWhitespace enabled code like this:
```
{{#each collection}}
<div>{{this}}</div>
{{/each}}
```
normally resulting this:
```
<div>ele1</div>

<div>ele2</div>

<div>ele3</div>
```

would be preprocessed to be like this:
´´´
{{#each collection}}<div>{{this}}</div>{{/each}}
´´´
resulting this:
```
<div>ele1</div>
<div>ele2</div>
<div>ele3</div>
```

This feels more natural and helps creating cleaner and more readable layouts, partials and page code.
Improved regular expressions

Fixed a few issues in regexp. Now it matches multiple styles of line endings and is not as greedy when removing whitespace. (```\s``` was matching line-endings when not intended)
Merge pull request #142 from Arkkimaagi/patch-2

Added a variable for excluding pages from build
Looks good to go, thanks for the contribution.
Merge pull request #143 from Arkkimaagi/patch-3

Added an option to remove extra whitespace around hbs tags
moving utils back into assemble but changing up the namespacing a little and adding tests
adding an engine factory