Fix for #11

Looks like I misread the API:
http://apidocs.mailchimp.com/api/2.0/campaigns/list.php It's not an
array of structs for the filterParam, just a struct.
Fixes for #10

The ints default to 0 when they are not nullable, which acts as a
filter.  This is bad.  Switched to nullable -- this should allow the
MailChimp API to interpret these optional fields as 'not present'.
Added wercker build support
Updated wercker build box
Updated wercker steps
Updated wercker config
Removed wercker support.  Added AppHarbor solution

AppHarbor solution does not have tests
Merge pull request #54 from DDtMM/master

Adding some mergeVars functions
Added 'making contributions' section to readme
Updated README

Attempted to improve readability
Updated README

Added mailchimp love
Update ListError.cs

The API error response wasn't de-serialising email and error message correctly. From the API docs:
    "errors": [
        {
            "email": {
                "email": "example email",
                "euid": "example euid",
                "leid": "example leid"
            },
            "code": 42,
            "error": "example error",
            "row": {
                "...": "..."
            }
        }
Merge pull request #58 from paulandrewcampbell/master

Update ListError.cs
Merge pull request #56 from DDtMM/master

Completed MergeVars implementation
Fix to CampaignFilter class. Optional flag 'uses_segment' was non-nullable.
Merge pull request #59 from dazaris/master

Fix to CampaignFilter class. Optional flag 'uses_segment' was non-nullabable bool
Reinstate MergeVar for explicit specification of lists/subscribe merge_vars param

Fixes #61

Re-instatement of the deleted MergeVar class for utilization on the Lists.Subscribe method.  Reasoning:

 - people who just want to use the API should have the most common fields available to them in this class... it also "self-documents" the lists/subscribe function, and makes it ready to use out of the box.

 - Interests/Groups are the recommended way to deal with multiple subscriber interests / mailings: http://kb.mailchimp.com/article/best-practices-for-lists#manage ("Maintain a single list per client or organization in your account.")  So utilizing at least the grouping parameters is (or should be) very commonplace.

 - Assuming you re-instate the MergeVar class, here is an approach that works for adding additional fields to the merge_vars parameter:

 1) When you create your arguments object, cast the MergeVar type as an object so it properly gets serialized in the MailChimpManager.Subscribe:

```C#
            //  Create our arguments object:
            object args = new
            {
                apikey = this.APIKey,
                //...
                merge_vars = (object)mergeVars,
                // cast to object so ServiceStack grabs the entire object
                // (IE if it is inherited with custom merge fields added),
                // not just the base type
                //...
            };
```

 2) Then simply inherit the object and add the fields needed:

```C#
        [System.Runtime.Serialization.DataContract]
        public class MyMergeVar : MergeVar
        {
            [System.Runtime.Serialization.DataMember(Name = "FNAME")]
            public string FirstName
            {
                get;
                set;
            }
            [System.Runtime.Serialization.DataMember(Name = "LNAME")]
            public string LastName
            {
                get;
                set;
            }
        }
```

 3) Then just pass an instance of your inherited class to Subscribe.
Merge pull request #62 from AndrewKoransky/master

Reinstate MergeVar for explicit specification of lists/subscribe merge_vars param
correct / add docs for subscribe with merge vars
Merge pull request #63 from AndrewKoransky/master

correct / add docs for subscribe with merge vars